Prior to the present invention, as shown by Holub, U.S. Pat. No. 3,325,450, polysiloxane imides useful as insulation for electrical conductors were prepared by effecting reaction between a diaminopolysiloxane and benzophenonedianhydride in the presence of a suitable organic solvent, such as dimethylformamide, N-methyl-2-pyrrolidone, cresol, etc. The initial reaction was generally carried out from room temperature to 150.degree. C. resulting in the production of an intermediate polyamide acid derivative. Thereafter, the solvent was removed from the resulting amide acid derivative by heating at temperatures of from about 150.degree. C. to 400.degree. C. to effect cyclization and formation of the imide structure.
A related procedure is shown by Greber, Polykondensationsreaktionen Bifunktioneller Siliciumorganischer Verbindungen, Journal fur praktische Chemie. Band 313, Heft 3, 1971, S. 461-483, J. A. Barth, Leipzig. Although the procedure of Greber is somewhat different from that shown by Holub, both Holub and Greber utilize a dipolar aprotic solvent, such as dimethylacetamide to form a solution of a silicone-polyamide acid from which films can be cast onto a substrate and further heating is required to effect the cyclization of the polyamide acid to the polyimide state.
Improved results in methods for making silicone-polyimides can be obtained by utilizing aromatic bis(ether anhydride) or the corresponding tetracarboxylic acid in combination with amino alkylene terminated polydiorganosiloxanes as shown, for example, by Takekoshi et al., U.S. Pat. No. 3,833,546 and Heath et al., U.S. Pat. No. 3,847,867, assigned to the same assignee as the present invention.
Although silicone-polyimides have long been recognized for their potential as a source for extrudable wire coating insulation, the flammability requirements of the wire coating industry has generally restricted the use of these materials. In addition to flame retardance, wire coating fabricators also favor extrudable wire coating insulation having at least 150% elongation at break when pulled laterally from a clamped portion of the extrudate along the wire surface. However, efforts to increase the elongation characteristics of silicone-polyimide by increasing the weight percent of silicone has generally been found to increase the flammability of the silicone-polyimide.
In copending application Ser. No. 760,792, now U.S. Pat. No. 4,690,997, it was found that silicone-polyimide utilizing aromatic bis(etheranhydride), an aminoalkylene-terminated polydiorganosiloxane having a critical block length was extrudable onto wire and exhibited an elongation percent of 150 or greater while satisfying UL-94 flammability requirements.
As taught in Ser. No. 760,792, now U.S. Pat. No. 4,690,997, wire coating industry requirements can be satisfied providing a critical relationship is maintained between the polydiorganosiloxane block length and the weight percent silicone which is preferably 25% to 45% by weight based on the weight of silicone-polyimide. Polydiorganosiloxane block lengths having an average value of about 20 diorganosiloxy units or less has been found to provide effective results, while a block length of about 5 to about 15 chemically combined diorganosiloxy units is preferred.
Flame retardant silicone-polyimides can be made by the method shown in copending application Ser. No. 760,792 by effecting reaction between amine-terminated polydiorganosiloxane or "siliconediamine" having the formula, ##STR1## aryldiamine having the formula, EQU NH.sub.2 R.sup.2 NH.sub.2, (2)
with substantially equal molar amounts of aromatic bisanhydride, preferably aromatic bis(etheranhydride) of the formula, ##STR2## and organic dianhydrides, as defined hereinafter, where R is the same or different C.sub.(1-14) monovalent hydrocarbon radical, or C.sub.(1-14) monovalent hydrocarbon radical substituted with radicals inert during intercondensation, R.sup.1 is a C.sub.(2-14) divalent hydrocarbon radical, or C.sub.(2-14) divalent hydrocarbon radical substituted with radicals neutral during intercondensation, R.sup.2 is a divalent C.sub.(6-14) arylene radical, R.sup.3 is a divalent C.sub.(6-30) arylene radical, and n is an integer having an average value of 3 to 20 inclusive and preferably 5 to 15.
R.sup.1 is preferably C.sub.(1-4) polyalkylene, and R.sup.3 is preferably a divalent arylene radical selected from the class consisting of ##STR3## X.sup.1 is a member selected from the class consisting of ##STR4## p is equal to 0 or 1, and y is an integer equal to 1 to 5 inclusive.
Experience has shown that although the silicone-polyimides made in accordance with the method of Ser. No. 760,792, now U.S. Pat. No. 4,690,997, can provide valuable flame retardant fire coating compositions, the degree of elongation and flexibility in terms of flexural modulus often does not satisfy the requirements of the wire coating industry.
The present invention is based on our discovery that a substantial improvement in silicone-polyimide flexibility can be achieved by making the silicone polyimide in step-wise manner instead of simultaneously intercondensing the aromatic bisanhydride with the aryldiamine and the silicone diamine. It has been found that simultaneous intercondensation of the amine anhydride reactants can result in the production of a high molecular weight random silicone-polyimide. However, sequential addition of the aromatic bisanhydride, siliconediamine and aryldiamine has been found to form "oligomeric imide" which preferably has a DP (degree of polymerization) of from about 1.5 to about 50 intercondensed aromatic bisanhydridesilicone diamine groups, or aromatic bisanhydridearyldiamine groups, and terminated with a member selected from the class consisting of intercondensed aromatic bisanhydride, aryldiamine, or siliconediamine.
The term "flexibility" when defining the properties of the silicone polyimide made in accordance with the practice of the present invention, can be correlated with a reduction in the flexural modulus of the silicone polyimide as a result of "sequential intercondensation", as distinguished from "random intercondensation" of the aromatic bisanhydride, the aryldiamine and the siliconediamine. It also has been found that an increase in the elongation percent often occurs when sequential intercondensation is used. The term "aromatic bisanhydride" hereinafter means aromatic bis(etheranhydride) of formula (3), a mixture of such formula (3) diahydride with pyromellitic dianhydride, or benzophenone dianhydride, or biphenyldianhydride, or one or more of the latter dianhydrides free of the formula (3) dianhydride. The term "diamine" hereinafter means either the amine-terminated polydiorganosiloxane of formula (1) or the aryldiamine unless the particular bifunctional amine is specifically identified.